The present invention is related to a biological-thermal process. More particularly the present invention relates to a biological-thermal process for the treatment of refuse. Further, the present invention relates to an installation for performing a refuse treatment process and to products obtained from that process.
Household refuse consists essentially of water and so-called total dry matter (TDM). The TDM in turn comprises as an essential content, so-called biologically degradable organic dry matter (ODM), the major portion of which is biologically degradable (ODMbiol).
The disposal of household refuse must comply with the general requirement that the mass to be finally disposed of in a dump must be as low as possible. Furthermore relevant legislation provides for limiting, in particular, the ODM content in the dump refuse. The separation of reusable material from the household refuse has been attempted to reduce the ODM mass to be dumped. The reduction of ODM has also been obtained in known processes by composting under aerobic conditions. The heat developed during composting is used for drying the material. Drying down to 15% by weight of residual water or, in other words, up to 85% of dry matter (DM) in the resulting final product can be obtained by an appropriate preparation of the material to be composted (e.g. by drying) and by adjustment to optimum parameters during composting (e.g., temperature). Absent a contrary indication, percentages that are given in this specification are to be understood as by weight.
In known installations, composting is preceded by a fermentation step, i.e. an anaerobic fermentation, in order to generate biogas. Generally, this biogas is combusted at the site in a heating and power station attached to the installation. The energy recovered, generally thermal and electric energy, is used for internal purposes and may also be utilized otherwise.
An object of the present invention is to obtain a further reduction of the amount of the final products of a refuse treating installation which must be finally disposed of in an expensive manner.
Another object of the present invention is to increase the dry matter fraction of the final products which are to be finally disposed of.
A further object of the present invention is to increase, during refuse treatment, the content or the creation of those products which can be disposed of without problems and/or which are reusable.
At least one of the above indicated objects is implemented by a biological-thermal process for the treatment of refuse in which an essential portion of the refuse, which is accessible to fermentation, is subject to anaerobic fermentation, and the fermentation residue thus obtained is dried, without further composting, until a dry matter content of at least about 90%, and more preferably 95%, is obtained. The dry material is subjected to screening and/or sifting in order to separate it into fractions of different particle sizes, different materials and/or different specific gravity or mass.
According to the process, household refuse is fed into a fermenter after having passed usual preparation steps, such as comminuting, metal separation, screening, etc. The operating conditions of the fermenter are selected in such a manner that the fermentation in the fermenter is conducted until any fermentation activity in the fermentation residue has dried away (exhaustive fermentation), thus obtaining a maximal decomposition of the ODMbiol.
Further treatment methods, which are per se known, may be applied to the fermentation residue, for example a separation under pressure in order to achieve a separation into a solid fraction and a liquid fraction.
The fermentation residue is subjected to a drying step, preferably together with other refuse components previously removed, until a dry matter DM content of at least about 90% is attained. The energy necessary for this step is preferably taken from the combustion of the biogas formed during fermentation. Generally, a sufficient amount of biogas is generated such that the thermal portion of the electric power generation from a fueled power station, e.g. a block-type thermal power station, is obtained by combusting the biogas.
Preferably, the fermented material has a content of biologically degradable dry matter of at most 30% by weight of the proportion present prior to fermentation.
Preferably, the fermentable portion of the refuse obtained by pre-sorting, or the total amount of the refuse, is subjected to a separation process, preferably in an extruder press having a final operating pressure of at least about 700 bar, by manual sorting, by mechanical sorting, by screening or a combination of methods, in order to a separate the refuse into a solid matter portion and a pulp portion, the pulp portion containing essentially all of the biologically degradable organic substance content which is subjected to anaerobic fermentation.
Preferably the biogas formed during the anaerobic fermentation is combusted, preferably in a block-type thermal power station, producing sufficient electric energy for the drying process.
Preferably, a liquid fraction is mechanically separated from the fermentation residue, the liquid fraction being concentrated to a dry matter content of at least 35% by vaporization of water. Preferably, prior to the concentration process the pH of the liquid fraction is adjusted, to a neutral or slightly acidic value, and more preferably to a value in the range of from 5 to 6, in order to suppress the formation of gaseous ammonia during the concentration process.
In accordance with another aspect of the invention, there is provided a process for the treatment of a fine fraction of refuse having an upper particle size selected from within the range of from about 15 to 40 mm, the fine fraction being conveyed to a hydrosifter having at least one washing spiral, wherein the fraction having a higher specific gravity which settles at the bottom of the hydrosifter will be carried away by the washing spiral, and wherein washing liquid is charged into the output end of the washing spiral and runs through the washing spiral countercurrently to the conveying direction of the spiral in order to carry substances adhering to the refuse particles back into the hydrosifter.
Preferably, the condensate formed during the concentrating is used as the washing liquid in the washing spiral.
Preferably, polluted water from the washing of the fine fraction in the washing unit is used for humidifying the fermentable refuse portion in order to adjust the humidity content of the fermentable refuse portion to that needed for aerobic fermentation.
In accordance with yet another embodiment of the invention there is provided an apparatus for the treatment of refuse comprising a fermenter for carrying out an anaerobic fermentation, the fermenter being operable through an adjustment of operational parameters, such as dwell time and filling height, whereby material discharged from the fermenter is essentially not further biodegradable and in particular not further fermentable, and a drying device disposed downstream of the fermenter for drying at least the fermented matter to a dry substrate content of at least 90% whereby a product having essentially no biological decomposition activity is produced.
Preferably, at least one screening device and/or a sifter is provided down stream of the drying device for separating the dry, biologically inactive material into fractions according to specific gravity and/or particle size.
Preferably, the fractions comprise a fine particle fraction and a coarse particle fraction, the upper limit of the particle size of the fine fraction being selected within the range of from 2 to 10 mm.
In accordance with yet a further aspect of the invention, there is provided, the material comprising a dry substance content of at least 90% and preferably at least 95% and being biologically inactive.
In accordance with yet a further aspect of the invention, there is provided a material obtained by the process described above, wherein the material is granular, having a upper particle size in the range of from 15 to 40 mm, and having a maximum organic dry matter content of 5%, determined as its ignition loss.